Modern health care facilities require preparation and administration of very large numbers of parenteral solutions to patients. Such solutions include those administered for both nutritional and therapeutic purposes.
In recent years, positive displacement fluid infusion pumping devices have been developed for administration of parenteral solutions to patients. Such infusion pumping devices permit precise control of drug administration to a patient during a given period of time, and facilitate efficient and accurate solution administration.
U.S. Pat. No. 4,639,245, to Pastrone et. al., U.S. Pat. No. 4,88,186, to Pastrone et.al., and U.S. Pat. No. 4,842,584, to Pastrone, all of which are hereby incorporated by reference, disclose a positive displacement fluid infusion pumping device and components thereof, which have met with widespread acceptance by the health care industry. This pumping system includes a combination of a pump driver and an associated removable and disposable pump cassette. The pump cassette includes a self-contained positive displacement pump device, which is operated by a reciprocable pump plunger of the associated pump driver. The pump driver further includes selectively operable valve actuators, which cooperate with valve mechanisms provided in the pump cassette for accurate and highly automated administration and infusion of parenteral solutions.
Pending U.S. application Ser. No. 07/444,459, filed Dec. 1, 1989, discloses a solution pumping system generally of the above type, including a disposable pump cassette, and associated pump driver. The system of this pending application is particularly configured for automated compounding and preparation of parenteral solutions, for subsequent infusion to a patient. Portions of the above-identified application, not inconsistent with the present disclosure, are hereby incorporated by reference.
Solution pumping systems of the above infusion and compounding types employ preassembled, disposable pump cassettes. Such pump cassettes typically include a cassette body including juxtaposed front and rear body members, between which is positioned a membrane-like elastomeric diaphragm. The diaphragm cooperates with the front body member to provide valve mechanisms at various inlets and outlets defined by the front body member, with openings in the rear body member exposing the diaphragm for operation of the valve mechanisms by valve actuators of the associated pump driver.
Additionally, the front body member of the cassette defines a pump chamber which, together with the internal diaphragm, provides the self-contained positive displacement pump of the cassette. The rear body member defines an opening through which a reciprocable plunger of the associated pump driver is movable for operating the pump, whereby liquid can be pumped through the cassette.
As will be appreciated, the accurate and consistent pumping of liquids requires that during use, the various components of the cassette, including the internal diaphragm, remain in secure association with each other. This is particularly true in the region of the diaphragm which provides the pump mechanism, since this portion of the diaphragm is repeatedly deformed by the associated reciprocable pump plunger.
In the past, the components of such disposable cassettes have been subjected to compression during assembly, whereby the front and rear body portions of the cassette act to grip the internal diaphragm to retain it in the desired position and orientation. The components are then permanently joined together, such as by sonic welding of the body members, with the diaphragm thus retained in compression.
However, experience has shown that some constructions do not lend themselves to creating sufficient compression of the diaphragm attendant to sonic welding or other joining of the body members together. Additionally, this compression of the diaphragm can lead to "compression set" or "load decay", a cold-flow or creep-like phenomenon which can be exhibited by the elastomeric diaphragm material during storage of the cassette prior to use. As a consequence of this compression set, the initial degree of compression created on the diaphragm during cassette assembly can undesirably be lost.
The present invention contemplates avoiding the problems of compression set or load decay by assembling a pump cassette under relatively low compression, with the cassette being subjected to compression attendant to use so that high pumping pressures can desirably be achieved.